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Monday, July 24, 2017

A total solar eclipse is happening Aug. 21!

1. Solar eclipses don’t happen at every New Moon. The reason is that the Moon’s orbit tilts 5° to Earth’s orbit around the Sun. Astronomers call the two intersections of these paths nodes. Eclipses only occur when the Sun lies at one node and the Moon is at its New (for solar eclipses) or Full (for lunar eclipses) phase. During most (lunar) months, the Sun lies either above or below one of the nodes, and no eclipse happens.
2. Eclipse totalities are different lengths. The reason the total phases of solar eclipses vary in time is because Earth is not always at the same distance from the Sun and the Moon is not always the same distance from Earth. The Earth-Sun distance varies by 3 percent and the Moon-Earth distance by 12 percent. The result is that the Moon’s apparent diameter can range from 7 percent larger to 10 percent smaller than the Sun.
3. It's all about magnitude and obscuration. Astronomers categorize each solar eclipse in terms of its magnitude and obscuration, and I don’t want you to be confused when you encounter these terms. The magnitude of a solar eclipse is the percent of the Sun’s diameter that the Moon covers during maximum eclipse. The obscuration is the percent of the Sun’s total surface area covered at maximum. Here's an example: If the Moon covers half the Sun's diameter (in this case the magnitude equals 50 percent), the amount of obscuration (the area of the Sun's disk the Moon blots out) will be 39.1 percent.

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